This invention relates to portable grinders. In the past attempts to design a grinder guard which is effective in retaining the fragments of a broken grinder wheel have experienced varying degrees of success and few have been entirely satisfactory.
At the high rotation speeds of a modern hand held grinder, wherein the speeds often exceed 12,000 rpm, the task becomes extremely difficult for the grinder designer. Due to the high energy to be absorbed, it is extremely difficult to design a guard with sufficient strength to withstand the forces generated and at the same time light enough not to be an unnecessary burden to the operator, a safety related factor in itself. Several materials including high strength steels, aluminum and reinforced plastics have been tried with limited success. In some instances where the guards have been designed to absorb a great deal of energy a problem has existed in the mounting of the guard and guards have been separated from the grinder as mounting bolts have sheared off. This creates a further hazard.
Current safety guard requirements for portable grinders dictate wheel fragment containment must be maintained through a minimum of 180.degree. arc in the direction of the operator and that the guard must remain in protective position after wheel explosion.
Current cup guard configurations for Type 6 and specifically for Type 11 taper wheels, typically do not adequately contain wheel fragments. Wheel containment failures can be primarily attributed to loss of axial containment relative to the grinder arbor axis.
When wheel fragments begin to escape axially, effective peripheral containment through a minimum of 180.degree. of the guard envelope is completely lost. It has also been ascertained that any inside peripheral guard obstructions such as skirt mounting bolt heads or nuts, impede free particle escapement. These skirt mounting bolt heads or nuts additionally act as energy transfer points to the main guard mounting bolts. If the wheel speed at the time of explosion is sufficiently great, enough energy is expended to sheer the bolts under rigid mounting conditions.
The current state of the art for guard mounting provides for either rigid or integral guard mounting, or pinch bolt guard mounting. Both of these mounting configurations have their specific limitations. Rigid or integral guard mounting affords very little energy dissipation during a wheel explosion which normally will result in severe guard distortion and failure. Additionally, if the mounting bolts fail, the resulting guard rotation will leave the operator exposed to wheel fragments. The pinch bolt configuration affords energy dissipation. However, the resulting guard rotation will also leave the operator exposed to exploded wheel fragments.